The invention relates to parts made of ceramic matrix composite (CMC) material. The term “CMC material part” is used herein to mean a part made of a material comprising fiber reinforcement made of carbon or ceramic fibers that has been densified by a matrix comprising at least a majority of ceramic.
CMC material parts are used in various applications, in particular in the fields of aviation and space, where they are used because of their thermostructural properties, i.e. their capacity to constitute structural elements because of their mechanical strength, in particular in bending, in traction, and against impacts, which is much greater than that of solid ceramics, combined with their capacity to conserve this mechanical strength up to temperatures that are high, well above 1000° C.
In CMC materials, cracking of the matrix is inevitable in practice, often as from fabrication.
Proposals have been made to interpose an interphase coating between the fibers and the matrix, which coating is capable of transferring load effectively between the matrix and the fibers and is made of a material that is capable of deflecting cracks reaching the interphase coating so as to prevent cracks that are propagating in the matrix from reaching the reinforcing fibers and causing them to break, since that rapidly degrades the mechanical properties of the CMC material. Documents U.S. Pat. No. 4,752,503 and U.S. Pat. No. 5,026,604 disclose making an interphase out of pyrolytic carbon (PyC) or boron nitride (BN) having a structure that is lamellar or made up of lamellae. It is also known to make a crack-deflector interphase out of porous material.
Nevertheless, in utilization conditions under an oxidizing atmosphere and at high temperature, cracks propagating as far as the interphase provide paths giving access to oxygen, and the PyC or BN interphase can then oxidize, as indeed can the underlying fibers if they are made of carbon, thereby degrading the CMC material.
A first known approach for improving resistance against oxidation consists in making a sequenced interphase with alternating nanometric layers of crack-deflector material, such as PyC or BN, and nanometric layers of material having an oxidation protection function, in particular a material such as silicon carbide (SiC) or an Si—B—C ternary system capable, in the presence of oxygen, of forming a vitreous compound capable of healing cracks by passing to the pasty state at the high temperatures to which the CMC material is exposed. Reference may be made to document U.S. Pat. No. 5,738,951 which, for an SiC matrix composite material, describes forming such a sequenced interphase by pulsed chemical vapor infiltration (P-CVI) in which the elementary layers of the interphase have a thickness of less than 10 nanometers (nm).
Using P-CVI to form a sequenced interfaces of the (PyC-SiC)n type is also described in documents by Sébastien BERTRAND et al.: “Influence of strong fiber/coating interfaces on the mechanical behavior and lifetime of Hi-Nicalon/(PyC-SiC)n/SiC minicomposites”, Journal of the American Ceramic Society, Blackwell Publishing, MALDEN, Mass., US, vol. 84, N° 4, Apr. 1, 2011 (2001-04-01), pages 787-794, and by Roger NASLAIN et al.: “Processing of ceramic matrix composites by pulsed-CVI and related techniques”, Key Engineering Materials, Trans Tech Publications Ltd., STAFA-ZURICH, CH, Vol. 159-160, Jan. 1, 1999 (1999-01-01), pages 359-365.
In those two documents, the composites described are mini-composites or micro-composites with unidirectional reinforcement.
The document by Takashi NOZAWA et al.: “Tensile, flexural, and shear properties of neutron irradiated SiC/SiC composites with different fiber-matrix interfaces”, STP/ASTM International, ASTM International, WEST CONSHOHOCKEN, Pa., 2001, US, Vol. 1475 STP, Jan. 1, 2004 (2004-01-01), pages 392-404, also mentions mini-composites with unidirectional fiber reinforcement and an SiC matrix, together with an interphase of (SiC-PyC)n type, the composites being made by an isothermal CVI process, without further details.
A second known approach consists in incorporating, in the matrix, one or more faces of a material capable of conferring self-healing properties on the matrix so as to prevent or slow down the propagation of cracks within the matrix, where such phases are made in particular out of boron carbide B4C or out of an Si—B—C ternary system. Reference may be made to document U.S. Pat. No. 5,965,266, which describes a matrix made up of SiC phases alternating with phases of B4C or of Si—B—C.
A third known approach consists in making a sequenced matrix comprising layers of crack-defector material, e.g. PyC or BN, alternating with the layers of ceramic material, the deflection of cracks within the matrix slowing down the access of an ambient oxidizing medium to the interphase or to the fibers. Reference may be made to document U.S. Pat. No. 6,068,930.
A fourth known approach consists in making an interphase of PyC or of BN with little anisotropy in order to enable strong bonding with a matrix or a matrix layer made of ceramic, in particular of SiC, such that the breaking strengths in shear within the interphase layer and at the fiber-interphase and interphase-matrix bonds are greater than those encountered within the matrix.